Forensic Science
Introduction to DNA/RNA
A. Genetic information – instructions to produce specific protein molecules,
which function as structural materials, enzymes or other vital
biochemicals.
B. Genes – the portion of DNA molecules that contain the genetic
information; each gene contains the instructions to make one protein.
C. DNA molecules
1. Building blocks are formed of nucleotides joined so that sugar
and phosphate portions alternate.
2. The nucleotides in the sugar-phosphate alternating pattern form a
long polynucleotide chain that is the “backbone” of the DNA
molecule.
3. Organic bases project from the backbone and bond weakly by H
bonds to the bases of the second strand of the DNA molecule.
4. The organic bases of a DNA nucleotide can be one of four types:
a) Adenine (A)
b) Thymine (T)
c) Cytosine (C)
d) Guanine (G)
5. Adenine will only bond to thymine and cytosine will only bond
to guanine; this is called complementary base pairing.
6. A DNA strand with the base sequence ATGC could only join
with a second strand with base sequence TACG.
7. The sequence of the base pairs along a DNA molecule is what
encodes the genetic info that specifies a particular protein’s
amino acid sequence.
8. The ladder-like DNA molecule twists to form a double helix and
may be millions of base pairs long.
9. Human Genome Project is effort to map out the total genetic
information within a human cell. This includes about three
billion DNA nucleotides, which form approx. 60,000 to 70,000
protein-encoding genes. This info helps identify genes that
underlie many illnesses, which can lead to new diagnostic tests
or treatments.
10. Except in identical twins, the sequence of DNA nucleotides is
unique to each person. This fact allows DNA fingerprinting to
identify individuals as the source of blood, semen or other
forensic evidence.
D. Genetic code
1. This is the method of storing information for protein synthesis on
a DNA molecule.
2. Each of 20 different amino acids is represented in a DNA
molecule by a sequence of three nucleotides.
3. The sequence TAC on a DNA molecule indicates where to start
synthesis of a protein.
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Forensic Science
Introduction to DNA/RNA
4. The sequence ATT on a DNA molecule indicates where to stop
synthesis of an amino acid.
5. Amino acids joined in a particular sequence form specific
proteins.
E. RNA (ribonucleic acid)
1. RNA differs from DNA in several ways:
a) RNA is single-stranded
b) Nucleotides contain ribose sugar instead of deoxyribose.
c) Instead of thymine nucleotides, RNA molecules contain
uracil (U) nucleotides.
2. Messenger RNA (mRNA) carries info in a gene’s nucleotide
sequence from the nucleus to the cytoplasm.
3. Synthesis of mRNA begins when the enzyme RNA polymerase
associates with the DNA base sequence at the beginning of a
gene (the instructions for synthesizing a particular protein).
4. At this site, other enzymes unwind and pull apart the doublestranded DNA molecule.
5. RNA polymerase moves along the separated strand, exposing
other portions of the gene and stringing together (polymerizing) a
molecule of mRNA from nucleotides complementary to the
unwound DNA strand.
6. RNA polymerase somehow “knows” which of the two strands of
DNA hold the information, and which way to read the DNA.
7. When the RNA polymerase reaches the special DNA base
sequence that is the termination signal, the enzyme releases the
newly formed mRNA molecule and leaves the DNA.
8. The process of copying DNA info into the structure of mRNA is
transcription.
9. The triplet of nucleotides in mRNA that specifies a particular
amino acid is called a codon.
10. Once mRNA is formed, it moves out of the nucleus into
cytoplasm and associates with ribosomes to act as pattern for
synthesis of protein molecules in process called translation.
F. Protein synthesis
1. Requires that correct amino acids are present in cytoplasm and
positioned in proper location along strand of mRNA.
2. Transfer RNA (tRNA) correctly aligns amino acids to form
proteins.
3. 20 different types of amino acids to form biological proteins = 20
different types of tRNA to serve as guides.
4. anticodon – three contiguous nucleotides of a tRNA which are
complementary to a specific mRNA codon.
5. tRNa reads mRNA, brings complementary amino acid together
where joined by peptide bonds in correct sequence, in close
association with ribosomes.
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Forensic Science
Introduction to DNA/RNA
6. Enzymes assoc with ribosomes control the addition of amino
acids to form protein, which folds into its unique shape as it is
formed.
7. mRNA and tRNA can function repeatedly.
G. DNA replication
1. DNA molecules are replicated (duplicated) during interphase of
cell cycle so that each newly formed cell has copy of parent
cell’s genetic info when cell divides.
2. This process begins when bonds between complementary base
pairs of the double strands in DNA molecule break.
3. Double-stranded structure pulls apart and unwinds.
4. New DNA nucleotides form complementary pairs with the
exposed bases and enzymes knit together the sugar-phosphate
backbone.
5. The new strand of complementary nucleotides form along each
of old strands, forming two complete DNA molecules, each with
one old strand of original DNA molecule and one new one.
6. Errors can occur as DNA is replicated; cells have mechanisms to
detect these errors (mutations) and correct them. Sometimes
these fail, may produce illness. Mutations may occur
spontaneously or be induced by mutagens, such as radiation and
toxic chemicals.
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